The present invention relates to a color picture device and, more particularly, to an improvement in a convergence device for performing convergence correction of three electron beams emitted from an electron gun assembly having a delta arrangement.
A color picture device of the type mentioned above is described in, for example, Japanese Patent Publication No. 25577/1971. This color picture device comprises an envelope having a neck section, a funnel section and a glass panel section with a phosphor screen. Electron guns are arranged in a delta form in the neck section, and emit three electron beams of red, green and blue. The centers of these electron guns define vertices of a regular triangle, and one vertex defines the vertical axis of the faceplate of the glass panel section. The electron beams emitted from the electron guns are respectively converged to a predetermined aperture in a shadow mask by electron lenses consisting of a plurality of grid electrodes and a common convergence electrode mounted on the distal ends of the three electron guns. A pair of substantially parallel pole pieces are arranged in the convergence electrode so as to surround each corresponding electron beam. The three electron beams passing through the aperture land on R, G and B phosphor dots on the phosphor screen formed on the inner surface of the faceplate of the glass panel, so that the phosphor dots emit light of the corresponding color.
In order to display a clear image on the phosphor screen, the respective electron beams must land on the corresponding phosphor dots on the entire screen, and the three electron beams must be correctly converged on the phosphor screen so as to prevent color misregistration. Various parts are arranged around the envelope so as to prevent a convergence error. Blue lateral magnet, purity magnets, convergence yokes, and deflection yokes are arranged around the neck section sequentially in the propagation direction of the beams. First, convergence of R, G and B beams on the screen is corrected in the radial direction by stick magnets assembled in the convergence yokes. Second, convergence is corrected by the blue lateral magnet in the horizontal direction of the blue beam. The purity magnets adjust the deflection center of the three electron beams so as to improve color purity. The convergence yokes are arranged around the neck section so as to oppose pairs of pole pieces of the convergence electrode. The convergence yokes serve to correct a convergence error which is caused when the electron beams are deflected at the center of the screen by the deflection yokes.
The function of the convergence yokes and pole pieces for the electron beams will be described below. Lines of magnetic force generated from one magnetic pole (N pole) of the convergence yoke are absorbed by the wide magnetic field absorption portion of the pole piece and are guided therethrough to the end portion of the pole piece. The distance between the end portions of the pair of pole pieces is kept substantially constant, so that the magnetic field generated between the end portions of the pair of the pole pieces by the corresponding convergence yoke is uniform. The magnetic path returning to the other magnetic pole (S pole) of the convergence yoke is in adverse order of that described above. The magnetic field of the convergence yoke which is generated between the end portions of the pair of pole pieces provides a slight deflection to the electron beam in the radial direction of the neck. The amounts of the slight deflection in the vertical and horizontal directions is controlled in synchronism with the vertical and horizontal deflections, respectively. Voltages are applied to the vertical and horizontal convergence coils to optimally converge R, G and B beams. In this manner, the convergence yokes correct misconvergence occurring at the peripheral portion of the screen.
However, such a color picture device cannot be used in a display device requiring high precision, e.g., for a high-precision character or graphic display device or for a high-quality TV display. In such a display device, the spot diameter of an electron beam converged on the phosphor screen must be reduced in order to obtain high resolution and sharpness. In order to reduce the spot diameter of the electron beam, the diameter of the electron lens must be increased and magnification M of the lens must be reduced. In this case, the diameter of the electron beam which passes through the electron lens and becomes incident on the convergence electrode increases with the degree being inversely proportional to the magnification M of the electron lens. Therefore, the distance between the pair of pole pieces must be increased in accordance with the diameter of the electron beam. When the pole piece distance is increased, sensitivity of the convergence yoke is degraded. In addition, when misconvergence in the peripheral portion of the screen is corrected by the convergence yoke, the beam on the screen is distorted so that the convergence correction direction coincides with the major axis of the beam spot. The distortion amount, i.e., the ratio of the major axis to the minor axis of the beam spot is proportional to the convergence correction amount by the convergence yoke. According to an experiment, even if the magnetic field generated between the pair of pole pieces has a uniform shape, the beam spot is distorted on the screen due to the edge effect of the magnetic field. Therefore, although the diameter of electron lens increases and the resolution is improved at the center of the screen, resolution near the peripheral portion of the screen is significantly degraded.